Polyvinyl alcohol-based fiber having excellent hot water resistance and production process thereof

ABSTRACT

A high-strength and highly wet-heat-resistant polyvinyl-alcohol-based fiber--in which the crosslinking agent has hardly been oxidized by the heat at the drawing time upon preparation of the fiber, the crosslinking agent has not exhaled so much at the time of dry heat drawing, and the crosslinking agent has penetrated even inside of the fiber so that not only the surface but also the inside of the fiber has sufficiently been crosslinked--can be obtained by the steps of: preparing a polyvinyl-alcohl-based fiber by spinning the polyvinyl-alcohol-based solution, wet drawing the fiber, applying an acetalization compound of an aliphatic dialdehyde having at least 6 carbon atoms to the fiber, subjecting the fiber which contains above compound to dry heat drawing to a total draw ratio of at least 15, and then crosslinking the drawn filament with an acid under mild crosslinking treatment conditions.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a polyvinyl alcohol (hereinafter abbreviatedas "PVA")-based fiber which has excellent hot water resistance becauseit has been sufficiently crosslinked not only on the fiber surface butalso inside of the fiber. In particular, this invention is concernedwith a PVA-based fiber which, owing to sufficient crosslinkage eveninside of the fiber, hardly causes the dissolution of PVA from the endsurface of the fiber and at the same time has a sufficient strength,when subjected to dyeing treatment in a hot water bath at a hightemperature, or when subjected to steam curing in a high-temperatureautoclave to heighten the strength of a cement product to which thefiber has been added as a reinforcing fiber.

BACKGROUND ART

A PVA-based fiber has the highest strength and the highest modulus ofelasticity among general-purpose fibers and also has good adhesivenessand alkali resistance so that it has attracted attentions particularlyas a cement reinforcing material substitutable for asbestos. It is,however, poor in hot water resistance (which will be also called "wetheat resistance") so that its applications have so far been limited evenif it is employed as general industrial materials or materials forclothes. For example, when the PVA-based fiber is used for a cementproduct as a cement reinforcing material, it is accompanied with theproblem that it cannot be subjected to autoclave curing at hightemperature conditions. In the case where a PVA-based fiber is employedas a reinforcing fiber for a cement product, it is now the common butinevitable practice to subject the product to autoclave curing underheating conditions at room temperature or a low temperature. Theautoclaving at such a low temperature also involves problems such asinsufficiency in the size stability and strength of the resulting cementproduct and requirement for long curing days.

When the PVA-based fiber is used for mixed fabric products with apolyester-based fiber, a dyeing method commonly employed for the dyeingof a polyester fiber, in which dyeing is carried out in an aqueoussolution at a high temperature of from 120° C. to 130° C. using adisperse dye, cannot be applied because of inferior hot water resistanceof the PVA-based fiber. So, the use of the PVA-based fiber for clotheshas been limited largely also from this viewpoint.

A carbon fiber has been used in some cases for autoclave curing at hightemperatures but it is accompanied with a problem that it has inferioradhesiveness with cement matrix and thus produces only poorreinforcement effect and at the same time is expensive.

Attempts have been made to improve the wet heat resistance of aPVA-based fiber. For example, Japanese Patent Application PublicationNo. Sho 30-7360/1955 or Japanese Patent Application Publication No. Sho36-14565/1961 describes that a PVA-based fiber is made hydrophobic bythe crosslinking reaction (formalization) of hydroxyl groups of PVA byusing formalin and that the fiber available by this method hassufficient hot water resistance against dyeing or washing. Such aPVA-based fiber, however, has not hot water resistance high enough tomeet the level required by the present invention, that is, hot waterresistance high enough to withstand high-temperature autoclave curingand moreover, it has a disadvantage in low strength.

Japanese Patent Application Laid-Open No. Sho 63-120107/1988 discloses aprocess which comprises formalizing a high strength PVA-based fiber. Thefiber obtained by this process has however a formalization degree as lowas 5-15 mole % and only very small part of the amorphous region of thefiber has been rendered hydrophobic so that the fiber available by thismethod has not sufficient hot water resistance and therefore cannot beused at all as an industrial material exposed in repetition to wet heatfor a long period of time or as a cement reinforcing material subjectedto high-temperature autoclave curing.

In Japanese Patent Application Laid-Open No. Hei 2-133605/1990(corresponding to European Patent No. 351046 and U.S. Pat. No.5,283,281) or Japanese Patent Application Laid-Open No. Hei1-207435/1989, disclosed is a method in which hydroxyl groups of PVA arecrosslinked by incorporating an acrylic-acid-based polymer in aPVA-based fiber or a method in which hot water resistance is improved byimparting an organic peroxide, isocyanate compound, urethane compound,epoxy compound or the like to the fiber surface, thereby crosslinkinghydroxyl groups of PVA. The crosslinking reaction using an acrylicacid-base polymer is not successful because the crosslinkage formed byan ester bond readily hydrolyzes by an alkali in the cement and theacrylic-acid-based polymer loses its effect, while the latter methodalso involves a problem that during autoclave curing or when exposed inrepetition to wet heat, swelling or dissolution starts appearing fromthe central region of the fiber, because the crosslinkage has occurredonly on the surface of the fiber.

In addition, a method of improving wet heat resistance by conductingdehydration crosslinking using an acid is disclosed in Japanese PatentApplication Laid-open No. Hei 2-84587/1990 or Japanese PatentApplication Laid-open No. Hei 4-100912/1992. As a result of anadditional test by the present inventors, however, it has been foundthat an attempt to conduct crosslinking even inside of the fiber causessevere decomposition of a PVA-based fiber, leading to the eminentlowering in the fiber strength.

The crosslinkage by a dialdehyde compound is clearly described inJapanese Patent Application Publication No. Sho 29-6145/1954 or JapanesePatent Application Publication No. Sho 32-5819/1957. According to theabove description, the post treatment is conducted in a mixed bathcontaining a dialdehyde compound and, as a reaction catalyst, an acid,but the dialdehyde compound does not easily penetrate into the inside ofthe high strength PVA-based fiber having highly oriented andcrystallized fiber molecules. It is therefore difficult to effectcrosslinking inside of the fiber.

Japanese Patent Application Laid-Open No. Hei 5-163609/1993 discloses aprocess which comprises imparting a dialdehyde compound to a spinningfiber, conducting dry heat drawing at a high draw ratio, and treatingwith an acid, thereby causing crosslinkage inside of the resultingfiber. The specific examples of the dialdehyde compound described in theabove literature include aliphatic dialdehyde compounds and aromaticdialdehyde compounds each having 6 or less carbon atoms. When analiphatic dialdehyde having less carbon atoms is employed, thedialdehyde compound imparted to the spinning fiber is exhaled therefromat the time of dry heat drawing and does not remain in the fibersufficiently, leading to a problem that there does not exist sufficientcrosslinkage (intermolecular crosslinkage) between PVA-based moleculeswhich is effective for the attainment of hot water resistance. The useof an aromatic-based dialdehyde, on the other hand, is also accompaniedwith the problem that because it is an aromatic compound, it causessteric hindrance and prevents easy penetration into the fiber, andmoreover lowering in the strength tends to occur. The above-disclosedmethod therefore cannot satisfy the both requirements for hot waterresistance and high strength. In the above publication, it is describedthat when a dialdehyde compound having high reactivity is employed, itmay be acetalized with an alcohol and as a representative example, acompound obtained by acetalizing malondialdehyde (an aliphaticdialdehyde having 3 carbon atoms) with methanol, that is,tetramethoxypropane is given. A dialdehyde compound having highreactivity generally has small carbon atoms such as malondialdehyde.Accordingly, an acetalization product of such a dialdehyde compound isaccompanied with the problems that it tends to be exhaled from the fiberat the time of dry heat drawing, similar to the above case of aaliphatic dialdehyde compound, so that sufficient crosslinkage cannot beformed and moreover, in the case of the dialdehyde compound having smallcarbon atoms, intramolecular crosslinking tends to occur whileintermolecular crosslinking necessary for the improvement of the heatresistance does not occur readily.

Finding that a PVA-based fiber which has been crosslinked even itsinside and has excellent hot resistance can be obtained by having adialdehyde compound, which is described in the above Japanese PatentApplication Laid-Open No. Hei 5-163609/1993, penetrate into the insideof a PVA-based fiber which has been subjected to dry heat drawing, andimmersing the resulting fiber in a bath containing a monoaldehyde and acrosslinking catalyst, thereby causing a crosslinking reaction; and thatthe PVA-based fiber so crosslinked can withstand autoclave curing at160° C., the present applicant filed a patent. It is laid open asJapanese Patent Application Publication No. Hei 5-263311/1993(corresponding to European Patent No. 520297 and U.S. Pat. No.5,380,588). The above process surely makes it possible to produce aPVA-based fiber which has been crosslinked even its inside and hasexcellent hot water resistance. The process however causes a problemthat since the dialdehyde compound has been imparted to the PVA-basedfiber after the completion of dry heat drawing, that is, after thecompletion of its crystal orientation, the dialdehyde compound does notpenetrate into the inside of the fiber sufficiently and when the fiberso obtained is subjected to autoclave curing at 170° C. or higher, thefiber will dissolve out.

In short, the processes known to date are accompanied with the followingproblems. In the case of the process in which a crosslinking agent isadded to a fiber prior to dry heat drawing, that is, a fiber whosecrystals have not been oriented yet, to have the crosslinking agentpenetrate into the inside of the fiber, the crosslinking agentintentionally penetrated is exhaled from the fiber or is oxidized at thetime of the subsequent dry heat drawing step and sufficient crosslinkingreaction does not occur. While, in the case where a crosslinking agentis added after dry heat drawing, the crosslinking agent cannot penetrateinto the inside of the fiber easily and sufficient crosslinkage is notformed inside of the fiber.

DISCLOSURE OF THE INVENTION

The present invention relates to a process capable of maintaining highstrength of a fiber, causing intermolecular crosslinkage, which iseffective for the improvement of hot water resistance, even inside ofthe fiber, substantially preventing the oxidation of a crosslinkingagent caused by the heat at the time of dry heat drawing, and reducingthe exhalation of the crosslinking agent at the time of drawing; andalso a PVA-based fiber having high strength and high hot waterresistance available by the method.

The present inventors have found that a PVA-based fiber having hot waterresistance and high strength, which it has been impossible to produce byconventional techniques, can be produced by using a specific dialdehydecompound as a crosslinking agent and effecting crosslinking by aspecific method, and completed the invention.

The present invention therefore provides a PVA-based fiber which hasbeen crosslinked by an acetalization product of an aliphaticpolyaldehyde having at least 6 carbon atoms and having an internalcrosslinking index (CI) and tensile strength (DT) that can satisfy thefollowing equations (1)-(3):

    CI≧86.5-2×10.sup.-6 ×(DT).sup.5.8       ( 1)

    CI≧75                                               (2)

    DT≧5 g/d                                            (3)

The present invention also provides a process for producing a PVA-basedfiber, which comprises the steps of:

preparing the PVA-based fiber by spinning a solution of a PVA-basedpolymer and then wet drawing,

applying an acetalization product of an aliphatic polyaldehyde having atleast 6 carbon atoms contain to the PVA-based fiber,

subjecting the resulting fiber to dry heat drawing to give a tensilestrength of 10 g/d or higher,

and then treating in a bath of an aqueous sulfuric acid solutionsatisfying the following equation (4)

    137/C.sup.0.05 -52≦T≦137/C.sup.0.05 -32      (4)

wherein C means a sulfuric acid concentration (g/l) of the bath of anaqueous sulfuric acid solution and T means a treating temperature (°C.).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the relation between an internalcrosslinking index (CI) and a tensile strength (DT) of a fiber as willbe defined later in the present invention. In the diagram, the slashedportion corresponds to the scope of the present invention. In FIG. 1,also described are the values of the crosslinked PVA-based fiberavailable by the process disclosed in Japanese Patent ApplicationLaid-Open No. Hei 5-263311/1993 (corresponding to European Patent No.520297 and U.S. Pat. No. 5,380,588) and the value of the crosslinkedPVA-based fiber available by the process disclosed in Japanese PatentLaid-Open No. Hei 2-133605/1990 (corresponding to European Patent No.351046 and U.S. Pat. No. 5,283,281). From the results, it can beunderstood that the fiber according to the present invention has by farhigh internal crosslinkage and has excellent hot water resistancecompared with the above-described crosslinked PVA-based fibers.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will hereinafter be described in further detail.

The term "a PVA-based polymer" as used here in means a PVA-based polymerhaving a viscosity-average polymerization degree of at least 1500 and asaponification degree of at least 98.5 mole %, preferably 99.0 mole %.The higher the average polymerization degree of the PVA-based polymeris, the more the tying molecules linking between the crystals and theless the number of the terminals of the molecules (an increase in thenumber of the molecules is disadvantageous). A higher averagepolymerization degree therefore makes it possible to attain highstrength, high modulus of elasticity and high hot water resistance ofthe fiber and is therefore preferred. The average polymerization degreeof at least 1700 is particularly preferred, with at least 2000 beingmore preferred. It is however difficult, in general, to prepare aPVA-based polymer having a polymerization degree exceeding 30000 andsuch a polymer is therefore not suited from the viewpoint of theindustrial production.

The present invention also embraces, as PVA-based polymers, thosemodified by a modification unit such as ethylene, allyl alcohol,itaconic acid, acrylic acid, maleic anhydride or a ring-opened productof maleic anhydride, arylsulfonic acid, fatty acid vinyl ester such asvinyl pivalate or vinyl pyrrolidone, or the above-described ionic grouppartially or wholly neutralized. The modifying unit may be used in anamount of 2 mole % or smaller, with 1 mole % or smaller being morepreferred.

For spinning of the PVA-based polymer, the PVA-based polymer is firstdissolved in a solvent and then defoamed, whereby a spinning dopesolution is prepared. Examples of the solvent usable here includepolyhydric alcohols such as glycerin, ethylene glycol, diethyleneglycol, triethylene glycol and butanediol, dimethyl sulfoxide,dimethylformamide, diethylenetriamine and water; and mixed solvents ofat least two of them. Particularly, dimethyl sulfoxide, or a polyhydricalcohol such as glycerin or ethylene glycol is preferred because at thetime when the spinning dope solution in such a solvent is poured in acoagulation bath, a uniform gel structure is formed and as a result, ahigh-strength fiber can be obtained.

To the spinning dope solution in which the PVA-based polymer has beendissolved in a solvent, it is possible to add boric acid, a surfactant,a decomposition inhibitor, various stabilizers, a dye and/or a pigment.Additives which impair the spinning property or drawing property arehowever not preferred.

The PVA-based polymer concentration in the spinning dope solution ispreferably 5-50 wt. %. For the wet spinning method or dry-wet spinningmethod, 5-20 wt. % is preferred, while for the dry spinning method,10-50 wt. % is preferred. As a temperature of the spinning dopesolution, generally employed is 100°-230° C.

The spinning dope solution so obtained is spun in accordance with anyone of the wet, dry and dry-wet method, followed by coagulation. In thewet or dry-wet spinning method, the spinning dope solution is coagulatedinto a fiber in a coagulation bath. Examples of the solution for thecoagulation bath include alcohols such as methanol or ethanol, ketonessuch as acetone, methyl ethyl ketone or methyl isobutyl ketone, aqueousalkali solutions and aqueous solutions of an alkali metal salt, andmixtures thereof. To form a uniform gel structure by graduallyconducting solvent extraction upon coagulation and thereby to attainhigher strength and hot water resistance, it is preferred to add asolvent constituting the spinning dope solution to said coagulation bathsolution in an amount of at least 10 wt. % and then mix them. Inparticular, it is preferred to use a 9:1 to 6:4 (weight ratio) mixedsolvent of an alcohol represented by methanol and the solvent for thespinning dope. To obtain a gel having a uniform microcrystallinestructure, that is, a high strength fiber, it is also preferred toreduce the temperature of the coagulation bath solution to 20° C. orlower, whereby the spinning dope solution discharged is quenched. It ismore preferred to lower the temperature of the coagulation bath solutionto 10° C. or lower to render the coagulation filament more uniform.

To prevent fusion adhesion between fibers and facilitate the subsequentdry heat drawing, it is desired to conduct wet drawing of so coagulatedfiber at a draw ratio of at least 2 in a solvent-containing state. Whenthe coagulation bath solution is an aqueous alkali solution or containsan alkali, neutralization under tension is preferred prior to wetdrawing. Examples of the extracting medium employed in the next solventextraction include primary alcohols such as methanol, ethanol andpropanol; ketones such as acetone, methyl ethyl ketone, methyl propylketone and methyl isobutyl ketone; ethers such as dimethyl ether andmethyl ethyl ether; and water. The fiber so extracted is then added witha lubricant as needed to dry the fiber. In the case of the dry spinningmethod, dry filament is produced by evaporating the solvent on and afterthe spinning time without using an extracting medium.

One of the greatest features of the present invention resides in that anacetalization product of an aliphatic dialdehyde having at least 6carbon atoms is used as a crosslinking agent and such an acetalizationproduct is added to a spinning filament in any one of the steps fromspinning to drying to have the acetalization product penetrate into theinside of the spinning filament. Even by heating upon dry heat drawing,the acetalization product of an aliphatic dialdehyde having at least 6carbon atoms is not exhaled much from the inside the fiber and itremains inside of the fiber after drawing, thereby bringing aboutcrosslinkage sufficient to permit hot water resistance being able towithstand to autoclave curing at 170°-180° C. When such an acetalizationproduct is added after drying the fiber, however, it does not easilypenetrate into the inside of the fiber owing to a large molecular weightof the acetalization product, and crosslinkage occurs only on thesurface of the fiber so that it is difficult to obtain a fiber havingsatisfactory hot water resistance.

In the present invention, based on the above-described findings, anacetalization compound of an aliphatic dialdehyde having at least 6carbon atoms, said acetalization compound having a larger molecularweight compared with that of the conventionally employed crosslinkingagent, is used as a crosslinking agent. Such a crosslinking agent isadded to a spinning filament in any one of the steps from spinning todrying. As a result, and also owing to the specific crosslinkingconditions which will be described later, it becomes possible to obtaina PVA-based fiber being able to withstand autoclave curing at 170°-180°C., which it has been impossible to produce by the conventionaltechnique.

In the present invention, a particularly preferred method for impartingan acetalization product to the fiber is to add the acetalizationproduct to an alcohol or ketone of extraction bath to dissolve theformer in the latter and have the acetalization product to penetrateinto the swollen-state filament which is just passing through theextraction bath. By this method, the acetalization product can penetrateinto the inside of the fiber easily. In the present invention, it isaccordingly preferred to employ, as a spinning method, a wet spinningmethod using an extraction bath or dry-wet spinning method.

Examples of the acetalization product of an aliphatic dialdehyde havingat least 6 carbon atoms in the present invention include compounds eachobtained by reacting a dialdehyde having at least 6 carbon atoms such ashexanedial, heptanedial, octanedial, nonanedial, decanedial,2,4-dimethylhexanedial, 5-methylheptanedial or 4-methyloctanedial withan alcohol such as methanol, ethanol, propanol, butanol, ethylene glycolor propylene glycol to acetalize both ends or one end of the dialdehyde.The acetalization product has preferably a boiling point of 230° C. orhigher, more preferably 260° C. or higher. When an aliphatic dialdehydehaving more than 14 carbon atoms is used, crosslinking reaction does notoccur easily and besides, orientation of the molecules is disturbed sothat high strength cannot be attained. Such a dialdehyde is nottherefore preferred. When an aliphatic dialdehyde has 5 carbon atoms orless, on the other hand, the acetalization product is exhaled at thetime of dry heat drawing and a sufficient amount of the acetalizationproduct does not remain inside of the fiber, which makes it impossibleto prepare a PVA-based fiber having sufficient hot water resistance.Furthermore, in the case of such a dialdehyde, the acetalization productchanges into its acid by the oxidation at the time of dry heat drawingand the resulting acid decomposes PVA or serves as a catalyst for thecrosslinking reaction, thereby causing crosslinking reaction upon dryheat drawing, whereby the smooth drawing of the spinning filament isprevented and therefore sufficient strength cannot be attained. Whensuch aliphatic dialdehydes outside the above range are used, the objectof the present invention cannot be attained.

By the use of an acetalization product of a dialdehyde other than analiphatic dialdehyde, for example, an acetalization product of anaromatic dialdehyde, the object of the present invention cannot beattained, because in this case, the steric hindrance prevents easypenetration of the acetalization product into the inside of the fiberand tends to induce a lowering in strength. If the product which has notbeen acetalized, that is, dialdehyde itself is used, a similarphenomenon as in the above case occurs. Described specifically,dialdehyde is oxidized into a corresponding carboxylic acid at the timeof heat drawing and the resulting carboxylic acid decomposes PVA orcauses a crosslinking reaction at the drawing time, which makes itdifficult to conduct drawing at a high draw ratio and therefore toprepare a high strength fiber. The use of dialdehyde itself involvesanother problem in odor, because it is prone to exhale at the dry heatdrawing time.

As described above, in the case when an aliphatic dialdehyde is used, itis oxidized by heat and oxygen under the dry heat drawing conditions andconverts into a corresponding carboxylic acid, causes partialcrosslinkage at the drawing time and fixes intermolecules of PVA,whereby a desired draw ratio cannot be attained and high-strength fibercannot be obtained. Furthermore, smoke and/or decomposition gas emittedat the dry heat drawing time contaminates the working environment andbecomes a problem. When the end group has been acetalized, oxidationhardly occurs at the dry heat drawing time and no such problems asdescribed above arise. Particularly an acetalization product of analiphatic dialdehyde having at least 6 carbon atoms is thermally stableand, different from the above case, is almost free from the exhalationof the dialdehyde at the time of dry heat drawing. Compared with the useof an aliphatic dialdehyde having at least 6 carbon atoms as acrosslinking agent, the use of its acetalization product enables thepreparation of a high strength fiber having at least 1 g/d higher thanthe case of the non-acetalization product, though depending on thepolymerization degree of the PVA-based polymer.

Specific examples of the particularly preferred acetalization product ofan aliphatic dialdehyde having at least 6 carbon atoms include1,1,9,9-tetramethoxynonane available by the reaction of 1,9-nonanedialwith methanol and 1,9-nonanedial-bisethylene acetal available by thereaction of 1,9-nonanedial with ethylene glycol. These acetalizationproducts are excellent in that they can prevent lowering in the strengthof the fiber and form intermolecular crosslinkage effective forattaining hot water resistance. Among these compounds, those having bothterminals acetalized are markedly stable against heat and thereforepreferred.

In the present invention, the acetalization product is adhered to adry-heat drawn filament in an amount of 0.3-10 wt. %, preferably 0.7-6wt. %. When an amount is smaller than 0.3 wt. %, hot water resistancebecomes insufficient owing to a low crosslinking density. An amountexceeding 10 wt. %, on the other hand, disturbs molecular orientation orpromotes the decomposition of a PVA-based polymer, thereby tending tocause a lowering in the strength.

When the PVA-based fiber is used as a reinforcing fiber forhigh-temperature curing FRC, a spinning filament which contains theacetalization product and has already been subjected to drying treatmentis subjected to dry heat drawing at a temperature not lower than 220° C.but not higher than 260° C., preferably not lower than 240° C. but nothigher than 255° C. and at a whole draw ratio of at least 15, preferably17 or higher. The term "whole draw ratio" as used herein means a valueexpressed by the product obtained by multiplying the draw ratio of wetdrawing conducted prior to drying treatment by that of dry heat drawing.At a whole draw ratio less than 15, a high-strength fiber which is theobject of the invention cannot be obtained. The drawing is carried outpreferably at a wet draw ratio of 2-5 and at a dry heat draw ratio of3-10.

Incidentally, for a PVA-based polymer having a higher polymerizationdegree, it is preferred to conduct dry heat drawing at highertemperatures. Temperatures exceeding 260° C., however, cause melting ordecomposition of the PVA-based polymer so that they are not preferred.High strength as required for FRC is not needed when it is applied toclothes, but it is necessary to heighten the crosslinking degree andalso to provide hot water resistance so that the resulting fiber canwithstand the high-temperature dyeing in a free state (that is a statewherein the fiber can shrink freely). In this case, the drawingtemperature is reduced by 5°-10° C. from the above-described one, bywhich the whole draw ratio becomes lower and molecular orientation andcrystallization are suppressed. As a result, crosslinking tends toproceed more readily, and a fiber having markedly high hot waterresistance can be provided.

The thus-drawn fiber containing the acetalization product of analiphatic dialdehyde having at least 6 carbon atoms has a tensilestrength of 10 g/d or higher. A tensile strength lower than 10 g/d isnot preferred because the tensile strength of the fiber largely lowersby the crosslinking treatment which will be conducted later. Morepreferred is the case where the fiber has a tensile strength of 12 g/dor higher. In addition, the thus-drawn fiber containing theacetalization product of an aliphatic dialdehyde having at least 6carbon atoms has preferably heat of crystal fusion of 130 joule/g orlower as measured by a differential thermal analysis. Sincecrystallization and orientation have generally proceeded in the highstrength fibers, heat of crystal fusion tends to become high. Similarlyin the case of the PVA-based fiber, high strength fiber has high heat ofcrystal fusion. The high strength PVA-based fiber has generally heat ofcrystal fusion of 135 joule/g or higher. The value of 130 joule/g orlower as specified in the present invention is slightly lower than thatof the conventional high strength PVA-based fiber. It means that in thepresent invention, it is preferred to conduct crosslinking treatmentwith a PVA-based fiber having lower heat of crystal fusion than that ofthe conventional high strength PVA-based fiber. More preferred is avalue not higher than 125 joule/g but not lower than 80 joule/g. APVA-based fiber can be imparted with excellent hot water resistance bysubjecting such a PVA-based fiber having low heat of crystal fusion tocrosslinking treatment and thereby forming intermolecular crosslinkagesufficiently even inside of the fiber.

Described specifically, crosslinking treatment is conducted by immersinga drawn fiber, which contains the acetalization product of an aliphaticdialdehyde having at least 6 carbon atoms, in a bath of an aqueoussulfuric acid solution for 5-120 minutes. By this method, the reactionoccurs between the hydroxyl group of the PVA-based polymer and theacetalization product, whereby intermolecular crosslinkage appears.Incidentally, the relation between the concentration (g/l) of sulfuricacid in the bath and the treating temperature (bath temperature) shouldsatisfy the following equation (4):

    137/C.sup.0.05 -52≦T≦137/C.sup.0.05 -32      (4)

wherein C means a sulfuric acid concentration (g/l) of the bath of anaqueous sulfuric acid solution and T means a treating temperature (°C.).

Treating temperatures (T) lower than 137/C⁰.05 -52 prevent sufficientprogress of crosslinking, while those higher than 137/C⁰.05 -32 bringabout a large reduction in the strength. More preferred is the casewhich satisfies the following equation (5):

    137/C.sup.0.05 -48≦T≦137/C.sup.0.05 -35      (5)

Concerning the relationship between the sulfuric acid concentration andtreating temperature as defined above in (4), either the sulfuric acidconcentration or the treating temperature is lower than theconventionally and industrially adopted conditions for the acetalizationof a PVA-based fiber. In the process according to the present invention,conditions different from the conventional ones are adopted as describedabove. It is possible to obtain a PVA-based fiber which has beencrosslinked sufficiently even its inside and has surprisingly excellenthot water resistance capable of withstanding autoclave curing at 170° C.or higher by adopting such conditions and using a special crosslinkingagent as described above. Furthermore, by the treatment with sulfuricacid at a high temperature and low concentration within a range asdefined above in (4) makes it possible to prepare a fiber being able towithstand even dyeing at 120° C. in a free state. Incidentally, uponcrosslinking treatment, sulfuric acid and formalin may be added to causeformalization at the same time. Moreover, a small amount of zincchloride or a surfactant may be added to promote crosslinking.

In the present invention, it is desired to conduct the above-describedcrosslinking treatment after cutting the fiber into a predeterminedlength, for example, 15-100 mm in the case where the fiber is used as astaple and 2-15 mm in the case where the fiber is used as a short-cutfiber for reinforcement of cement, in order to heighten the hot waterresistance of the fiber. When the fiber is cut after crosslinking, thecrosslinking degree of the cut surface becomes lower than that of thecircumferential portion of the fiber so that there is a fear of PVAdissolving out from the cut surface under severe wet heat conditions.The crosslinking treatment after cutting, on the other hand, does notcause the dissolution of PVA from the cut surface even under severe wetheat conditions, because sufficient crosslinking similar to theperipheral surface of the fiber is effected on the cut surface.

The PVA-based fiber obtained in accordance with the above methodsatisfies the following (1)-(3) at the same time.

    CI≧86.5-2×10.sup.-6 ×(DT).sup.5.8       (1)

    CI≧75                                               (2)

    DT≧5 g/d                                            (3)

wherein CI represents an internal crosslinking index and DT represents atensile strength of fiber.

If the resulting PVA-based fiber can satisfy neither (1) nor (2), it isvery difficult for the fiber to withstand autoclave curing at 170° C. orhigher or dyeing treatment at 120° C. in a free state. If it cannotsatisfy the above equation (3), it loses the characteristics as aPVA-based fiber in the application to cement reinforcement where highstrength is required or to clothes and consequently, it is of no utilityvalue. The PVA-based fiber satisfying the following equations (6)-(8) ismore preferred.

    CI≧90-2×10.sup.-6 ×(DT).sup.5.8         (6)

    CI≧80                                               (7)

    DT≧5 g/d                                            (8)

Particularly the PVA-based fiber tends to cause shrinkage or dissolutionby a dyeing treatment in a free state so that CI≧90 is desired. When thefiber is fixed in a cement as in autoclave, strength high enough tosatisfy both equations of CI≧80 and DT≧14 g/d is preferred. It ishowever difficult to industrially produce a fiber which can satisfy bothequations of CI>99 and DT>25 g/d.

The PVA-based fiber of the present invention which has been crosslinkedis preferred to have heat of crystal fusion not higher than 105 joule/gas measured by differential thermal analysis. The value not higher than105 joule/g means that the fiber has been crosslinked sufficiently anduniformly. When the heat of crystal fusion is higher than 105 joule/g,crosslinkage does not proceed into the inside of the fiber, which lowersits hot water resistance. More preferred is 100 joule/g or lower. Afiber having heat of crystal fusion lower than 50 joule/g is accompaniedwith the problem that its shrinkage factor in hot water increases, sothat 50 joule/g or higher is preferred.

The PVA-based fiber available by the present invention can be used forhigh-temperature curing FRC, general industrial materials for whichwater resistance is required and clothes which can be subjected tohigh-temperature dyeing.

The present invention will hereinafter be described in detail byexamples and comparative examples, in which all designations of "%" and"part" or "parts" mean wt. % and part or parts by weight unlessotherwise specified. Values of various physical properties in thepresent invention are those measured according to the following methods.

1. Viscosity-average polymerization degree (P) of a PVA-based polymer

The specific viscosity (η sp) of each of five diluted aqueous solutionsof a PVA-based polymer at 30° C. is measured in accordance with JISK-6726. The intrinsic viscosity η! is determined from thebelow-described equation (9) and the viscosity-average polymerizationdegree (P) is calculated in accordance with the below-described equation(10).

Incidentally, drawn uncrosslinked fiber is pressure dissolved in waternot lower than 140° C. to give a concentration of 1-10 g/l. If the fiberis not dissolved completely and there appears a small amount of a gelledsubstance, the gelled substance is filtered off through a 5 μm glassfilter and the viscosity of the resulting filtrate is measured. Inaddition, the concentration of the aqueous solution at this time iscalculated using a correction value obtained by subtracting the weightof the remaining gelled substance from the weight of the sample.

     η!=lim(c→0) η sp/c                         (9)

    P=( η!×10.sup.4 /8.29).sup.1.613                 (10)

2. Content of an acetalization product of an aliphatic dialdehyde

The content of an acetalization product of an aliphatic dialdehyde isdetermined by dissolving a drawn uncrosslinked filament in deuterateddimethylsulfoxide not lower than 140° C. and calculating the peak arearatio of the acetalization product to the CH₂ group peak of thePVA-based polymer by NMR.

3. Internal crosslinking index (CI)

About 1 g of a sample is cut to 6 mm and weight W₁ is weighed. The cutsample is put into a pressure stainless pot, together with 100 cc of anaqueous solution of artificial cement (an aqueous solution in which 3.5g/l of KOH, 0.9 g/l of NaOH and 0.4 g/l of Ca(OH)₂ have been dissolved).The pot is hermetically sealed, followed by treatment at 150° C. for 2hours. The residue is collected by filtration through a filter paper,followed by drying. The weight W₂ of the residue is weighed and CI iscalculated in accordance with the following equation:

    CI=W.sub.2 /W.sub.1 ×100

4. Heat of crystal fusion: Δ H (joule/g)

About 10 mg of a sample are weighed and charged in a open-type containerin a free state. The measurement is conducted using "DSC-2C type" (tradename; product of Perkin Elmer Co., Ltd.) from room temperature to 280°C. in a nitrogen gas atmosphere at a heating rate of 10° C./min and Δ H(joule/1 g of sample) is determined from the area of crystalline fusionendothermic peak.

5. Tensile strength of fiber (gram/denier: g/d)

In accordance with JIS L-1015, a single fiber which has beenmoisture-conditioned in advance is adhered to a mount to give a samplelength of 10 cm. It is allowed to stand at 25° C.×60% RH for 12 hours ormore. Using a chuck for 2 kg in Instron 1122, breaking strength (thatis, tensile strength) is determined at an initial load of 1/20 g/d and apulling rate of 50%/min. An average value of n≧10 is adopted. Concerningdenier (d), a single fiber is cut to 30 cm length under the load of 1/20g/d and the denier is determined from an average value of n≧10 asmeasured by the gravimetric method. Using the single fiber after themeasurement of denier, tensile strength is measured and the value of thetensile strength is corresponded to that of denier one by one. When thefiber is too short to be cut to 10 cm length, the maximum length is usedas a sample length and measured in accordance with the above-describedmeasuring conditions.

6. Autoclave resistance (wet bending strength WBS of slate)

A crosslinked PVA-based synthetic fiber is cut to 4-8 mm length. Using aHatschek machine, a mixture containing 2 parts by weight of the fiber, 3parts by weight of pulp, 38 parts by weight of silica and 57 parts byweight of cement is wet formed into a plate, which is subjected toprimary curing at 50° C. for 12 hours and then autoclave curing underany one of the following conditions: at 150° C. for 20 hours, 160° C.for 15 hours, 170° C. for 15 hours and 180° C. for 10 hours, whereby aslate is prepared. The slate so obtained is immersed in water for 24hours and then tested for bending strength in a wet state according toJIS K-6911.

7. Stable temperature (°C.) against hot water

Under no stretch, about 1 g of a crosslinked fiber or dishcloth andabout 200 cc of water are charged in a minicolor dyeing machine(manufacture of Techsum Giken Co., Ltd.), followed by heating to 110° C.over 30 minutes. After treating at 5° C. intervals from 110° C. to 130°C. for 40 minutes each, the condition of the fiber is macroscopicallyjudged and the maximum temperature of the fiber free from shrinkage orfusion adhesion between fibers is designated as stable temperatureagainst hot water.

EXAMPLES 1 AND 2 AND COMPARATIVE EXAMPLES 1 AND 2

PVA having a viscosity-average polymerization degree of 1,700(Example 1) or 3,500 (Example 2) and having a saponification degree of99.5 mole % was dissolved in dimethylsulfoxide (DMSO) at 110° C. to givea concentration of 15 wt. % (Example 1) or 11 wt. % (Example 2). Thesolution so obtained was discharged from a nozzle having 1000 holes,followed by wet spinning in a coagulation bath of 7° C. composed ofmethanol and dimethylsulfoxide at a weight ratio of 6:4. After wetdrawing to a draw ratio of 4 in a methanol bath of 40° C., almost allthe solvents were removed using methanol. To the final methanolextraction bath, 1,1,9,9-tetramethoxynonane, which had been obtained bymethoxylation of aldehydes at both ends of 1,9-nonanedial and had aboiling point of about 300° C., was added to give a concentration of 4wt. %/bath and the resulting mixture was made uniform. The fiber wasthen retained in the uniform solution for 1.5 minutes to have theacetalization product contain inside or on the surface of themethanol-containing fiber, followed by drying at 120° C. The filament soobtained was subjected to dry heat drawing in a hot-air oven formed ofthree sections at 170° C., 200° C. and 230° C. to a total draw ratio of17.2 in the case of Example 1 or dry heat drawing in a hot-air ovenformed of three sections at 170° C., 210° C. and 240° C. to a total drawratio of 17.5 in the case of Example 2, whereby a multi-filament ofabout 1800 denier/1000 filaments was obtained. The drawn filament wasthen immersed in a 70° C. aqueous solution of 20 g/l of sulfuric acidfor 30 minutes to cause crosslinking reaction (when C=20 g/l and T=70°C., 137/C⁰.05 =117.9° C.).

In Example 1 or 2, smoking and odor were hardly observed at the time ofdry heat drawing so that there were no problems at all in the workingenvironment.

In Comparative Example 1, in a similar manner to Example 1 except for1,9-nonanedial having a boiling point of about 240° C. was used insteadof 1,1,9,9-tetramethoxynonane, drawing was effected. As a result, thetotal draw ratio was reduced to 16.5, which was considered to be causedby the acidification of the solution of the methanol extraction bathowing to the conversion of a portion of 1,9-nonanedial into acorresponding carboxylic acid at the time of drawing. In addition,smoking and odor were observed at the time of drawing, which was aproblem in the working environment.

In Comparative Example 2, in a similar manner to Example 2 except that adrawn filament (total draw ratio of 17.5) free from1,1,9,9-tetramethoxynonane was used instead, multi-filament wasprepared. Then the filament was immersed in an aqueous solutioncontaining 100 g/l of formalin and 80 g/l of sulfuric acid at 80° C. for60 minutes to cause formalization reaction. For the evaluation using aslate, each crosslinked filament was cut to 6 mm.

Average polymerization degree and physical properties of the fibersobtained above in Examples and Comparative Examples are shown in Table1.

                                      TABLE 1                                     __________________________________________________________________________                   Ex. 1                                                                              Ex. 2                                                                              Comp. Ex. 1                                                                          Comp. Ex. 2                                   __________________________________________________________________________    PVA polymerization degree                                                                    1700 3500 1700   3500                                          Content of crosslinking agent (%)                                                            2.4  2.0  1.1    --                                            Heat of crystal fusion of fiber                                                              125  128  124    128                                           before crosslinking (joule/g)                                                 Tensile strength before                                                                      16.5 19.2 15.1   19.5                                          crosslinking (g/d)                                                            Tensile strength after                                                                       14.7 17.5 13.4   14.8                                          crosslinking (DT g/d)                                                         (DT).sup.5.8 (× 10.sup.6)                                                              5.89 16.2 3.45   6.13                                          Internal crosslinking index (CI)                                                             82.2 84.9 70.1   51.5                                          Heat of crystal fusion of fiber                                                              101  94   110    119                                           after crosslinking (joule/g)                                                  WBS     150° C.                                                                       294  340  270    191                                           (kg/cm.sup.2)                                                                         160° C.                                                                       266  328  195    *                                                     170° C.                                                                       225  319  *      *                                                     180° C.                                                                       160  261  *      *                                             __________________________________________________________________________     An asterisk (*) means that the value is less than 150 kg/cm.sup.2 and tha     the addition of a reinforcing fiber brought about no effect.             

EXAMPLE 3 AND COMPARATIVE EXAMPLE 3

A PVA-based polymer having a viscosity-average polymerization degree of8000 and a saponification degree of 99.9 mole % was dissolved inethylene glycol at 170° C. to a concentration of 8 wt. %. The solutionso obtained was discharged from a nozzle having 400 holes, followed byquenching and gelation in accordance with the dry-wet spinning method ina coagulation bath of 0° C. composed of methanol and ethylene glycol ata 7:3 ratio. After wet drawing at a draw ratio to 4 in a methanol bathof 40° C., almost all the solvents were removed by methanol. To thefinal methanol extraction bath, 1,9-nonanedial-bisethyleneacetal whichhad been obtained by acetalization of aldehydes at both ends of1,9-nonanedial with ethylene glycol and had a boiling point of about330° C. was added to a concentration of 8 wt. %/bath, which was thenmade into a uniform solution. The fiber was then retained in the uniformsolution so obtained for 2 minutes to have the acetalization compoundcontain inside and on the surface of the fiber, followed by drying at130° C.

The spinning dope so obtained was drawn to a total draw ratio of 19.4 ina radiation furnace formed of two sections at 180° C. and 248° C.,respectively, whereby a multi-filament composed of a 1000 d/400filaments having a viscosity-average polymerization degree of 8200 and acontent of the acetalization compound of 3.7% was obtained. After thedrawn filament was cut to 6 mm, it was immersed in an aqueous solutionof 75° C. (137/C⁰.05 =122.1) containing 10 g/l of sulfuric acid for 30minutes, whereby crosslinking reaction proceeded. The crosslinked fiberso obtained had an internal crosslinking index of 85.6 and a tensilestrength of 19.5 g/d (DT)⁵.8 =30.4×10⁶ !. Even by autoclave treatment at180° C., it had WBS of 295 kg/cm² and thus exhibited excellentperformance. In addition, at the time of thermal drawing treatment,there happened neither smoking nor odor so that the working environmentwas free of pollution.

In Comparative Example 3, in a similar manner to Example 3 except thatphosphoric acid was added to 0.05 wt. %/bath instead of1,9-nonanedial-bisethyleneacetal, dry heat drawing was conducted,whereby a fiber containing only acid crosslinkage was obtained. Thefiber so obtained had an internal crosslinking index of 47.8 and atensile strength of 16.9 g/d, which were much inferior to the results ofExample 3.

EXAMPLE 4 AND COMPARATIVE EXAMPLES 4-5

In a similar manner to Example 2 except that 1,1,6,6-tetramethoxyhexane(boiling point: about 350° C.) available by acetalizing aldehydes atboth ends of 1,6-hexanedial with methanol, was used instead of1,1,9,9-tetramethoxynonane in an amount of 5 wt. %, a crosslinked PVAfiber was obtained (Example 4). Also in this example, smoking and odorwere hardly observed at the time of dry heat drawing and there were noproblems at all in the working environment.

In a similar manner to Example 2 except that 1,1,3,3-tetramethoxypropane(boiling point: about 185° C.) available by acetalizing aldehydes atboth ends of malonaldehyde with methanol, was used instead of1,1,9,9-tetramethoxynonane in an amount of 5 wt. %, a crosslinked PVAfiber was obtained (Comparative Example 4).

In a similar manner to Example 2, except that1,1,5,5-tetramethoxypentane (boiling point: about 250° C.) available byacetalizing both ends of glutaraldehyde with methanol, was used insteadof 1,1,9,9-tetramethoxynonane in an amount of 5 wt. %, a crosslinked PVAfiber was obtained (Comparative Example 5).

Physical properties of the fibers obtained in those example andcomparative examples are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                       Ex. 4 Comp. Ex. 4                                                                             Comp. Ex. 5                                    ______________________________________                                        PVA polymerization degree                                                                      3500    3500      3500                                       Content of crosslinking agent (%)                                                              3.5     2.1       3.2                                        Heat of crystal fusion of fiber                                                                128     128       128                                        before crosslinking (joule/g)                                                 Tensile strength before                                                                        18.5    18.3      18.1                                       crosslinking (g/d)                                                            Tensile strength after                                                                         16.1    15.5      15.3                                       crosslinking (DT g/d)                                                         (DT).sup.5.8 (× 10.sup.6)                                                                9.99                                                         Internal crosslinking index (CI)                                                               83.9    71.1      72.5                                       Heat of crystal fusion of fiber                                                                98      115       110                                        after crosslinking (joule/g)                                                  WBS       150° C.                                                                           328     289     291                                      (kg/cm.sup.2)                                                                           160° C.                                                                           321     266     280                                                170° C.                                                                           306     209     210                                                180° C.                                                                           242     172     165                                      ______________________________________                                    

EXAMPLE 5

A completely saponified PVA having a viscosity-average polymerizationdegree of 4000 was dissolved in DMSO to a concentration of 12%. Thesolution so obtained was discharged from a nozzle having 400 holes andwas subjected to wet spinning in a coagulation bath of 7° C. composed ofmethanol and DMSO at a weight ratio of 7:3. After wet drawing to a drawratio of 4 in a methanol bath, almost all the solvents were removedusing methanol. To the final methanol extraction bath,1,1,9,9-tetramethoxynonane was added to a concentration of 5 wt. %/bathto have the acetalization product contain inside and on the surface ofthe fiber, followed by drying at 120° C. The spinning fiber so obtainedwas subjected to dry heat drawing to a total draw ratio of 16.0 in a hotair oven formed of three sections of 170° C., 200° C. and 235° C.,whereby a multi-filament composed of 1500 denier/400 filaments wasprepared. The drawn filament had heat of crystal fusion of 122 joule/g,tensile strength of 17.2 g/d and a tetramethoxynonane content of 3.9 wt.%. The drawn filament was then cut to 8 mm and crosslinking reaction wascaused by treating it with 80 g/l (80)⁰.05 =1.245! of sulfuric acid at70° C. for 20 minutes. The crosslinked filament so obtained had heat ofcrystal fusion of 90 joule/g, internal crosslinking index of 88.4 and atensile strength of 14.1 g/d (DT)⁵.8 =4.63×10⁶ !. After autoclavetreatment at 180° C., a high strength PVA-based fiber having WBS of 256kg/cm² and therefore having high wet heat resistance was obtained. Alsoin this example, smoking and odor were hardly observed at the time ofdry heat drawing and there were no problems at all in the workingenvironment.

EXAMPLE 6 AND COMPARATIVE EXAMPLES 6 AND 7

PVA having a viscosity-average polymerization degree of 1700 and asaponification degree of 99.5 mole % was dissolved in DMSO at 100° C. toa concentration of 17 wt. %. The solution so obtained was dischargedfrom a nozzle having 0.12φ mm×60 holes, followed by wet spinning in acoagulation bath of 10° C. composed of methanol and DMSO at a weightratio of 7:3. After wet drawing to a draw ratio of 3.5 in a methanolbath at 40° C., 1,1,9,9-tetramethoxynonane was added to a final methanolextraction bath to give a concentration of 2 wt. %/bath, followed bydrying at 120° C. The spinning dope so obtained was drawn to a totaldraw ratio of 10 in a radiation furnace formed of two sections of 170°C. and 200° C., respectively, whereby a multi-filament of 195 denier/60filaments was obtained. The drawn filament had heat of crystal fusion of115 joule/g, tensile strength of 12.6 g/d and a tetramethoxynonanecontent of 1.3 wt. %. The filament was then twisted at 80 T/m and then,in the form of a hank, charged in a minicolor dyeing machine (1.5)⁰.05=1.02! so that its bath ratio to a water dispersion containing 5 g/l oftetramethoxynonane, 1.5 g/l of sulfuric acid and 0.5 g/l of sodiumdodecylbenzenesulfonate be 1:50. After heating from 60° C. to 98° C.over one hour, crosslinking treatment was conducted at the temperaturefor 30 minutes, followed by washing with water and then drying at 60° C.The crosslinked filament was reduced in heat of crystal fusion to 81joule/g, and it had a CI of 91.8 which showed that the crosslinkageproceeded into the inside of the fiber. The tensile strength was loweredto 9.1 g/d (DT)⁵.8 =0.365×10⁶ !, however, it was found that the fiberwas usable for clothes at 120° C., which is a stable temperature againsthot water, under free stretch. Also in this example, smoking and odorwere hardly observed at the time of dry heat drawing and there were noproblems at all in the working environment.

In Comparative Example 6, in a similar manner to Example 5 except thatthe sulfuric acid concentration was changed to 20 g/l (137/20⁰.05=117.9) and the temperature of the treatment bath was changed to 98° C.,crosslinking treatment was conducted. In Comparative Example 7, insimilar manner except that the sulfuric acid concentration was changedto 10 g/l (137/10⁰.05 =122.1) and the temperature of the treatment bathwas changed to 110° C. crosslinking treatment was conducted. InComparative Example 6 where the sulfuric acid concentration wasrelatively high considering the physical properties of the fiber, CI was94.1 and tensile strength (DT) was 4.5 g/d. In Comparative Example 7where the treatment bath temperature was relatively high for thesulfuric acid concentration so that concerning physical properties ofthe fiber, CI was 95.2 and tensile strength (DT) was 3.8 g/d.

Capacity of Exploitation in the Industry

In the present invention, an acetalization product of an aliphaticdialdehyde having at least 6 carbon atoms, which is used as anacetalization agent, has a high boiling point so that exhalation, odoror thermal decomposition does not occur at the time of thermal drawing.By having the acetalization agent penetrate even into the inside of thefiber prior to thermal drawing and causing intermolecular crosslinkageunder relatively mild crosslinking treatment conditions after thermaldrawing, the PVA-based fiber can acquire high strength and excellent wetheat resistance, which it has been impossible to provide by conventionaltechniques.

The fiber according to the present invention can be used widely not onlyin the fields of general industrial materials such as rope, fishing net,tent or sheet for construction work, but also in the fields of areinforcing fiber for autoclave-cured cement which is subjected tohigh-temperature autoclave curing, and in the fields of a raw materialfor clothes which is mixed spun with a polyester fiber and is subjectedto high-temperature dyeing with a disperse dye or the like.

What is claimed is:
 1. A polyvinyl alcohol-based fiber which has beencrosslinked by an acetalization product of an aliphatic polyaldehydehaving at least 6 carbon atoms and having an internal crosslinking index(CI) and tensile strength (DT) which satisfy the following equations(1)-(3):

    CI≧86.5-2×10.sup.-6 ×(DT).sup.5.8       ( 1)

    CI≧75                                               (2)

    DT≧5 g/d.                                           (3)


2. A polyvinyl alcohol-based fiber according to claim 1, wherein theacetalization product of an aliphatic polyaldehyde having at least 6carbon atoms is an acetalization product of nonanedial.
 3. A polyvinylalcohol-based fiber according to claim 1, wherein heat of crystal fusionas measured by differential thermal analysis is 105 joule/g or lower.